.
Angewandte
Communications
within 30 min when the cis isomer is heated to 1008C. The
conditions for thermal isomerization reflect excellent thermal
stability of the cis isomer. No change in its e or the shape of
the spectra is observed after storage at room temperature for
2
weeks (in the dark). Throughout these isomerization
processes, only two species (Z and E isomers) were observed,
as indicated by a single isosbestic point at 446 nm.
Studies on the photochromic behavior in other solvents
were also carried out. In polar solvents such as acetonitrile
and methanol, Z!E isomerization can take place with lower-
energy light at 480 nm, while the reverse process still occurs
under white light but at a lower rate (Table S1). It is believed
that the solvent-polarity effect plays a significant role in the
isomerization rate as a result of a large difference in the
[
17]
dipole moments of the isomers.
For a sustainable application, a photoswitch should
possess an excellent photostability for consistent response
over multiple switching cycles. To investigate this feature, the
UV-Vis absorption of 4TCE in solution was monitored during
the alternate irradiation with blue (420 nm) and white light.
No significant photodegradation could be detected during 20
irradiation cycles (Figure S4), thus demonstrating the excel-
lent reversibility and photostability of this new photoswitch.
Quantum chemical calculations (ground-state DFT and
excited-state TDDFT) on both isomers were performed at the
Figure 4. Energy levels and corresponding frontier molecular orbitals
of both isomers calculated at B3LYP/6-31G* level.
constrained symmetry of the trans isomer also gives rise to
a greater relative oscillator strength for the S -to-S transition
0
1
(2.5 times larger than that of the cis isomer, in agreement with
experimental results). Overall, our calculations support the
feasibility of initiating isomer-selective photoisomerization in
4TCE by excitation through different electronic transitions.
In addition, DFT calculations show that the optimized
[18]
B3LYP/6-31G* level using the Gaussian09 package
in
order to better understand their electronic structures and
spectroscopy. The optimized ground-state geometries of both
isomers (Figure S5) are quite similar to those obtained by the
X-ray diffraction. UV-Vis spectra predicted from TDDFT
calculations coincide well with experimental results in terms
of spectral shape and relative absorption intensity (Figure 3a
inset). For the cis isomer, there are three significant electronic
ꢁ
1
ground-state energy of the trans isomer is about 4 kcalmol
lower than that of the cis isomer, which is in agreement with
the observation of the relative stability of the trans isomer in
the dark. However, the cis isomer is more stable under room
light, as discussed above. According to the theoretical analysis
and experimental results, we can attribute this observation to
the fact that the trans isomer has a larger oscillator strength
relative to the cis isomer in the visible region, particularly in
the green region of the solar irradiation spectrum, which
makes it easily excited by white light leading to isomerization
by way of a non-adiabatic transition in the excited state. It can
thus be predicted that green light will more effectively trigger
E!Z isomerization. To verify this hypothesis, a sample of the
trans isomer was irradiated with green light (515 nm) and
a complete conversion from trans to cis occurred in only 30 s.
On the contrary, the cis isomer does not have significantly
higher absorptivity compared to the trans isomer in the UV
and blue regions, which leads to incomplete Z!E isomer-
ization.
In summary, we have presented the promising photo-
switch molecule 4TCE, based only on reversible E/Z isomer-
ization. Its two isomers exhibit significantly different molec-
ular geometries leading to highly contrasting coplanarities,
effective conjugations, molecular dipole moments, end-to-end
distances, and symmetries. Spectral studies on pure isomers
showed a large separation between their lmax and a variation
in their relative absorption strengths. As a result, 4TCE can
be efficiently switched between its cis and trans forms by
irradiating with blue and white light, such that it is a useful
photoswitch with manageable photoisomerization, high con-
transitions, nominally, HOMO!LUMO (S ), HOMO-1!
1
LUMO (S ), and HOMO!LUMO + 1 (S ) transitions
2
4
(
Table S2). Therefore, the broad absorption bands from 380
to 550 nm can be assigned to the S and S transitions, whereas
1
2
the band around 320 nm is associated with the S transition.
4
Similar oscillator strengths of S and S transitions lead to
1
4
nearly the same absorption intensity of both bands. Photo-
excitation in one or both of these transitions triggers the
conversion from cis to trans.
Figure 4 illustrates these energy levels and corresponding
frontier orbitals. For the HOMO-1 and HOMO orbitals, the
electron distribution is largely located at the bisthienyl
moiety, whereas the LUMO exhibits substantial electron
density located near the cyano groups. This reflects an
electronic push–pull effect between the electron-donating
thiophene units and electron-withdrawing cyano groups. By
contrast, only one primary transition (HOMO!LUMO (S ))
1
occurs in the UV-Vis spectrum of the trans isomer and is
associated with the predominant absorption band at 520 nm.
Other transitions with very small oscillator strengths contrib-
ute to the weak band around 305 nm. The frontier orbitals
share qualitatively similar characteristics to those of the cis
isomer. Qualitative differences in the shapes of these orbitals
can be attributed to different molecular symmetry, where the
trans form imposed by C2h symmetry is more constrained than
the cis form with C (nominal C ) symmetry. The more
s
2v
4
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 1 – 6
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